For posting behind a lj cut, there's two ways:
If you use the Rich Text Editor (first tab) when you post, all you need to do is highlight the text you want to put behind the cut and click on the icon that looks like text with a squigly line under it next to the graphing icon. It will prompt you for the link text, which can be anything you want it to be, even the default. It does have its issues from time to time and might "double cut" if you try to undo it and do it again.
The second way is if you use the HTML Editor (second tab). This is the one you can use if you know some basic coding, and you would need to code it yourself if you want anything in the post to be bold or italics or whatnot. Basically, at the begining of the text you want to put behind the cut, you type [lj-cut text="whatever you want"] with the square brackets being replaced by the less-than (<) and greater-than (>) triangular brackets at the bottom of the keyboard. At the end of the text, you would type [/lj-cut], again replacing the brackets. Bolding is done by [b] and [/b] on either side of what you are bolding, and italics are done with [i] and [/i], again replacing the brackets.
Hopefully someone out there finds this information useful!